1. Field of the Invention
The present invention relates to a laser beam optical scanning system, and more particularly to a laser beam optical scanning system used as a write head of an image forming apparatus such as an electrophotographic copying machine, a laser printer or a facsimile.
2. Description of Related Art
Conventionally, in an electrophotographic laser printer, a laser beam optical scanning system for writing an image on a photosensitive drum has been using a laser diode as a light source. A laser beam emitted from the laser diode is deflected on a plane at a constant angular velocity by a deflector (polygonal mirror), and an optical element such as an f.theta. lens or an f.theta. mirror adjusts scanning speed. Then, the laser beam is imaged on a scanning surface (photo-sensitive drum). The laser beam emitted from the laser diode is a light diffused within a certain angle. In order to change the diffused light into a substantially parallel pencil of rays, a convergent lens (collimator lens) is disposed in front of the laser diode. Further, the parallel pencil of rays is imaged on a reflective facet in a linear form extending in the plane of deflection.
Recently, there is a trend of using resin instead of glass as a material of optical elements for the purposes of facilitating the manufacture and reducing the cost. However, a resin optical element (lens) changes its shape (thickness and radius of curvature) and refractive index according to temperature, thereby causing defocus and changing the size of a beam spot on the scanning surface.
Additionally, a light source unit comprising the laser diode, the convergent lens and a holder thereof changes its shape, thereby changing the convergence of the laser beam, according to temperature. The change is enlarged while the beam is passing through downstream optical elements, thereby causing defocus.